Polymers with Backbones of Heavier Group 14 Elements

4 Polymers with Backbones of Heavier Group 14 Elements 

1 Polysilanes. Following the first reports of soluble and processable polysilanes in the late 1970s, these macromolecules have attracted substantial interest from both fundamental and applied perspectives. The backbone of silicon atoms gives rise to unique electronic and optical properties as a result of the delocalisation of a-electrons. Several polysilanes have also been found to function as useful thermal precursors to silicon carbide fibres and these materials have also attracted attention with respect to microlithographic applications and as polymerisation initiators.  

One of the most remarkable features of the all-silicon backbone is that it leads to the delocalisation of a-electrons, a phenomenon which is essentially unknown in carbon chemistry.This can be understood in terms of the nature of the molecular orbitals associated with the Si-Si a-bonds. These are more diffuse than those associated with C-C a-bonds as they are constructed from higher energy 3s and 3p atomic orbitals. This leads to significant 

The main method used to synthesise polysilanes 10.7 involves the thermally induced Wurtz coupling reaction of organodichlorosilanes with alkali metals [eqn (10.7)]. Although improvements in this process have been reported e.g. by the use of ultrasound), the harsh conditions for this reaction tend to limit the side groups that can be successfully introduced to non-functionalised alkyl and aryl units and makes scale-up unattractive. 

Because of these limitations, considerable effort has been focused on the development of new synthetic routes to polysilanes. The early transition metal-catalysed dehydrocoupling process discovered in 1985 [eqn (10.8)] is potentially very attractive however, the molecular weights of the polysilanes formed to date are generally fairly low (Mn 8000). The catalysts used for these coupling reactions are usually titanocene or zirconocene derivatives.  

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